No abstract

No abstract

No abstract

Substances capable of markedly inhibiting the respiration, germination, and growth of tobacco seedlings were obtained after residues from timothy, corn, rye, or tobacco plants had been allowed to decompose under appropriate conditions in the soil. Bio-assay tests, based on manometric methods, made it possible to determine some of the conditions under which the decomposition gave rise to phytotoxic by-products and to obtain rapidly an accurate measure of the degree of their toxicity. Among the factors influencing the production of the toxic products, the species and stage of maturity of plant material added, the water content and pH of the soil, and length of decomposition period seemed most important. Substances capable of reducing the respiration of tobacco seedlings by 50 to 90% were consistently obtained when any of the abovementioned plant residues had been decomposing for 15 to 25 days under conditions of high soil moisture and when the pH of the soil solution was below 5.5 throughout this period. Timothy gave rise to substances possessing the highest toxic activity, followed by corn, then rye, and finally tobacco. Aqueous extracts of unamended soil or of macerated plant tissues prepared either before decomposition had taken place or when decomposition was inhibited by autoclaving the soil were not toxic.The toxic substances, obtained by water extraction, exhibited an inhibiting effect on respiration of tobacco seedlings after an exposure of less than one hour and also induced darkening and necrosis of root cells. Some extracts affected the cells of the apical meristem most severely while others affected only the cells of the elongation region.While the toxic substances have not been identified, they were relatively nonspecific in their action, affecting tobacco, timothy, and barley in approximately the same manner and in many instances possessed also antifungal activity. They were soluble in water, insoluble in petroleum ether, stable in acid, and most active in the pH range 4.5 to 5.8. They were precipitated by alkali and the activity was markedly reduced when readjusted to the acid range. They were also heat stable and did not lose their activity in storage at 1°–3 °C. provided they were covered by a layer of toluene.Because of their rapid production and the marked injurious effects exerted on various plants it is believed that these toxins may perform a significant role in the field as the primary cause of some root rots and in predisposing plants to attack by organisms not normally regarded as pathogenic.

Experiments were carried out to determine the role of Pratylenchus penetrans (Cobb, 1917) Filip. & Stek. 1941 in the peach replant problem. It is shown that this nematode is a true plant pathogen since it invades and causes necrosis of peach roots in the absence of bacteria and fungi. Necrosis of the root tissue occurs rapidly and appears to take place in advance of the invading nematode. The main mechanism involved in the formation of the lesion is the production of phytotoxic substances through hydrolysis of the cyanophoric β-glucoside, amygdalin. P. penetrans is capable of hydrolyzing this substance in vitro. It is concluded that, although P. penetrans is the most important primary parasite and a true plant pathogen, its main role in peach replant failure appears to be its ability to incite root degeneration by providing extensive infection courts for soil microorganisms.